Two major problems in dip coating hollow substrates that affect coating quality are burping and sucking. Burping refers to the release of gas from the substrate interior into the coating solution and is caused by volume expansion of the gas inside the substrate due to solvent evaporation in the substrate interior. Sucking refers to the entry of coating solution into the substrate interior and is caused by the volume reduction of the gas inside the substrate due to cooling. Conventional techniques for controlling the substrate interior pressure typically involve timed venting which uses a venting hole in the chuck assembly (holding the substrate) that is normally closed by a valve. The valve is opened once or more during the dip coating process for a certain period of time to relieve the pressure buildup inside the substrate to avoid burping. So the term timed means a set time to open the valve as well as the period of time it opens. Timed venting, however, does not address sucking and normally makes sucking worse if the valve is opened too long or prematurely opened. There is a need which the present invention addresses for new apparatus and methods which avoid or minimize the above mentioned problems.
Swain et al., U.S. Pat. No. 5,688,327; Chambers et al., U.S. Pat. No. 5,853,813; and Godlove et al., U.S. Pat. No. 5,683,755 disclose techniques for controlling the substrate interior pressure.
The present invention is accomplished in embodiments by providing an apparatus, to be used when an open end of a hollow substrate contacts a coating solution to define a solution free interior portion of the substrate, for controlling the level of the coating solution relative to the substrate interior, the apparatus comprising: a gas container capable of changing in volume; a channel connecting the gas container to the solution free interior portion of the substrate to allow gas flow in either direction between the gas container and the solution free interior portion; and pressure means for exerting a changeable, continuous pressure on the gas container that automatically exerts an increasingly greater pressure on the gas container as the gas container expands in volume and that automatically exerts a decreasingly lesser pressure on the gas container as the gas container decreases in volume.
In further embodiments, there is provided an apparatus, to be used when an open end of a hollow substrate contacts a coating solution to define a solution free interior portion of the substrate, for controlling the level of the coating solution within the substrate interior, the apparatus comprising: a gas container capable of changing in volume and having an elastic property that spontaneously exerts increasingly greater pressure on the contained gas as the gas container increases in volume and that spontaneously exerts decreasingly lesser pressure on the contained gas as the gas container decreases in volume; and a channel connecting the gas container to the solution free interior portion of the substrate to allow gas flow in either direction between the gas container and the solution free interior portion.
In other embodiments, there is provided a method, to be used when an open end of a hollow substrate contacts a coating solution to define a solution free interior portion of the substrate, for controlling the level of the coating solution relative to the substrate interior, the method comprising: establishing a channel to allow gas flow in either direction between a gas container capable of changing in volume and the solution free interior portion; and exerting a changeable, continuous pressure on the gas in the gas container that automatically exerts an increasingly greater pressure on the gas in the gas container as the gas container expands in volume and that automatically exerts a decreasingly lesser pressure on the gas in the gas container as the gas container decreases in volume.